1. Field of the Invention
The present invention relates to a system for controlling an air-fuel ratio in an internal combustion engine. The system includes a pair of specific concentration sensors, for example, a pair of oxygen sensors, positioned in an inlet side and an outlet side of a catalytic converter for detecting concentrations of a specific component in an exhaust gas for the purpose of feed-back control of the air-fuel ratio.
2. Description of the Related Art
A known double-oxygen sensor air-fuel ratio control system implements feed-back control of the air-fuel ratio with a first oxygen sensor positioned in an inlet side of a catalytic converter and a second oxygen sensor positioned in an outlet side of the catalytic converter. The second oxygen sensor in the outlet of the catalytic converter has a lower responsive speed but shows preferably little scatter in output characteristics. In the conventional double-oxygen sensor air-fuel ratio control system, some scatter in output characteristics of the first oxygen sensor can thus be eliminated according to the outputs of the second oxygen sensor, which effectively improves the accuracy in control of the air-fuel ratio.
The double-oxygen sensor air-fuel ratio control system executes an air-fuel ratio feed back control which balances the air-fuel ratio around a stoichiometric ratio through an integral control and a skip control according to output signals of the first oxygen sensor. During execution of the feed-back control, degrees of the integral control and the skip control are varied according to outputs of the second oxygen sensor. For example, a rich skip amount RSR for shifting the air-fuel ratio to the rich condition is adjusted according to the outputs of the second oxygen sensor.
The double-oxygen sensor air-fuel ratio control system, however, has such a problem that a time lag of lean and rich outputs from the second oxygen sensor due to the oxygen stored in the catalytic converter, that is, oxygen storage effects of the catalytic converter, undesirably lowers the accuracy of the air-fuel ratio control. To solve the problem, an improved air-fuel ratio control system has been proposed as disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. 63-195351. The improved system calculates a deviation of the output of the second oxygen sensor from a reference output corresponding to a stoichiometric air-fuel ratio, and increases an update quantity .DELTA.RS of the rich skip amount RSR per unit time in proportion to the increase in the deviation. This allows the air-fuel ratio to quickly approach to the stoichiometric ratio, thus compensating for a time lag of the outputs of the second oxygen sensor.
The inventors of the present invention have experimentally found a correlation of the outputs of the second oxygen sensor with the amounts of harmful substances contained in an exhaust emission as shown in FIG. 1. The correlation represents purification characteristics of the catalytic converter. As shown in FIG. 1, an output (voltage signal) SOX of the second oxygen sensor is within a predetermined range between a first voltage al (for example, 0.3 [V]) and a second voltage a2 (for example, 0.7 [V]) including a reference output level, there is relatively little emission of harmful exhausts, hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Exhausts of HC and CO drastically or exponentially increase when the output SOX exceeds the second voltage a2 while NOx abruptly increases when SOX becomes smaller than the first voltage al. Namely, when the output SOX of the second oxygen sensor is shifted from the predetermined range, harmful exhausts increase exponentially.
These findings show that ideal compensation characteristics of the air-fuel ratio compensation with respect to the output deviation of the second oxygen sensor are relatively small when the output SOX of the second oxygen sensor being within the predetermined range between a1 and a2 including the reference output level, and abruptly increase when the output SOX being out of the predetermined range as shown in FIG. 2. When the output SOX of the second oxygen sensor is within the predetermined range between al and a2 including the reference output level, small air-fuel ratio compensation preferably maintains a current desirable condition of reduced exhausts. When the output SOX of the second oxygen sensor is out of the predetermined range, on the contrary, abrupt increase in the air-fuel compensation quickly shifts the output SOX into the predetermined range between al and a2 for reduced emission of the exhaust gas.
The above conventional system, on the other hand, increases the rich skip amount RSR in proportion to the output deviation of the second oxygen sensor as shown by the two-doted chain line in FIG. 2. The compensation characteristics of the conventional system are compared with the ideal compensation characteristics described above. When the output SOX of the second oxygen sensor is within a certain range between b1 and b2 including the reference output level, which is wider than the predetermined range between a1 and a2, the air-fuel ratio is compensated excessively. When the output SOX is shifted from the certain range, on the other hand, the air-fuel ratio is compensated insufficiently. These problems of the conventional system result in the undesirable increase in the exhaust emission, the low drivability and the low fuel consumption.